A known type of LCD panel comprises a matrix of picture elements (pixels), each of which is located at the intersection of a strobe electrode and a data electrode. Displays of this type may use a ferroelectric liquid crystal (FLC) and, by applying suitable data and strobe signals, it is possible to display one or more grey levels having light transmissivity or reflectivity between minimum and maximum values which are referred to as black and white levels. This technique for achieving grey levels (by analogue means) may be used alone or in combination with spatial and/or temporal techniques for providing grey levels. It is thus possible to provide a large number of grey levels so as to increase the utility and range of applications of panel displays of this type.
A difficulty of achieving grey levels with displaces of this type is that the grey levels are sensitive to variations in temperature, panel thickness i.e. thickness of the liquid crystal layer, and pixel pattern i.e. variations in switching sensitivity of each pixel as a result of data signals applied to other pixels. In the absence of compensation, temperature variations cause the grey levels to vary from the levels expected of the data signals. Although techniques are known for compensating for large scale changes in temperature affecting the whole of a display panel, grey levels are so sensitive to temperature variation that small temperature variations over the display area can have a substantial effect on grey levels. Similarly, relatively small thickness variations in the liquid crystal layer over a small scale within the panel, for instance resulting from manufacturing tolerances, can have a substantial effect on grey levels. Such effects therefore limit the number of grey levels which can be reliably addressed and can have a detrimental effect on the quality of images displayed by the display.
JP-A-5-27719 discloses a ferroelectric liquid crystal display of the pixellated type. Each pixel is physically divided into two sub-pixels whose optical properties are individually controllable. In order to switch a pixel to a desired grey level, one of the sub-pixels is blanked to white and then switched from the white state so as to try to achieve the desired grey level. The other sub-pixel is blanked to black and then similarly switched so as to try to achieved the desired grey level. However, the effects of temperature variations and differences in thickness of the ferroelectric liquid crystal result in shifts of the function relating optical transmissivity to switching waveform. Both sub-pixels are affected in the same way but, because the sub-pixels are initially blanked to opposite states before being switched towards the desired grey level, the effects of such variations at least partially cancel out and the perceived grey level of the whole pixel is made substantially less dependent on variations, for instance in temperature or ferroelectric liquid crystal thickness. In particular, an effect which tends to make the grey level achieved by one sub-pixel darker than expected tends to make the other sub-pixel lighter than expected.
The arrangement disclosed in JP-A-27719 is capable of compensating for temperature variations and variations in liquid crystal thickness for bistable liquid crystals such as ferroelectric liquid crystals. However, this arrangement is unsuitable for non-bistable liquid crystals, which suffer from similar problems of sensitivity to temperature and liquid crystal thickness.